1980 International Scout I have a question

SCOTSHIGHLANDER

MEMBER
1980 INTERNATIONAL SCOUT
6 CYL
4WD
MANUAL
200,000 MILES

What I'd like to know is: Why do air filters have air horns?
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What bother's me is this: The intake diameter of the horn is far smaller than cylinder size. And yes, it is a stock air filter housing.
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I would have figured that the volumetric efficiency of the engine would be far improved without that air restriction.
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On high performance engines, the air filter is completely free of external obstructions such as to allow max air intake.
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So, why the air horn on stock automobiles? What's the deal there?
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By the way, the engine is a Nissan Diesel SD33T.
The 'T' stands for 'turbocharged.'
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Additionally, and just an additional thought, it would seem to me that with the air restriction the turbo would have to work just that much harder to move air.
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I thank you in advance for your time!

4 Replies

MHPAUTOS

MECHANIC
31,937 POSTS

Hi there,

Thank you for the donation,

Well I will just toss in my 2 bobs worth, and leave it open for others to comment on if they choose, for my money the air horn was a device to try and draw in cooler air, modern engines duct induction air from out side the engine bay, cooler air increases the volumetric efficiency of the engine, with older cars the air horn was always placed away from the exhaust side or there abouts on V8, efficiency was not a big issue when fuel was cheap, now days its all about efficiency, I have done a few mods over the years and cold air ducted many under bonnet induction systems as long as you can get a good flow with minimal turbulence things will work better for you. Also you must take into account that your truck is nearly 30 year old technology, so much has advanced since those very simple days of motoring.

Mark (mhpautos)

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Saturday, April 25th, 2009 AT 4:57 AM

SCOTSHIGHLANDER

MEMBER
3 POSTS

I thank you for that response, and it does seem to address something of the matter I raised.
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As an aside, the air filter on the Scout II diesels is on the left side of the engine (driver's perspective) above the intake/exhaust manifolds, and connects directly to the turbo.
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The radiator, which is larger than for the standard in-line 6, or slant 4 (both gasoline), is right in front of and just below the air horn "” about 8 inches away.
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So any air must arrive from that source, or from below that point (the roadway).
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My question however was why the air horn has such as small inside diameter such as to actually restrict the volume (quantity) of air.
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Mind you, that small opening doesn't in any way seem to affect the operation of the engine that I am able to determine. In all the years I've owned the vehicle (since new) I never detected an air starvation problem, at least that I was able to discern.
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Now I will suppose that upon further cogitation here, someone might ask: Well, if the intake valve opening isn't much larger, then why even ask?
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But of course that would neglect to consider that the total train of air isn't dependent upon just one intake valve, and in fact must consider all six operating at high speed.
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I wish I could pick the mind of one of the Nissan engineers who knows about that engine!
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I'm just curious as to what the possible effect might be with a larger opening, or if I should even attempt such. I wouldn't think that it would matter, inasmuch that the exit opening of the air filter (going to the turbo) is almost =3 times= the size of the ID of air horn!
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But again: I thank you for taking the time to reply!

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Saturday, April 25th, 2009 AT 10:46 AM

MHPAUTOS

MECHANIC
31,937 POSTS

Hi there,

I suppose that there are some questions that we all would like the answer too, and an engineers ear would always be handy. Lol, as for the size of the air opening at the horn, with a larger opening, the air flow would be slower, and no extra air would be ingested, the volume of air is proportional to the swept volume of the cylinders, plus the effect of the turbo boost, presume that the volume of are required at peek RPM must be available through the air horn, I have seen extra holes bored into the side of the air cleaner ass, the only thing that this would achieve would be less air speed at the air horn, as long as your air is filtered the opening will be of no consequence, remembering that the governor determines fuel delivery at any given engine rpm,
as you may or may not know, if a governor fails a diesel will over rev to destruction if the air flow is not restricted, or fuel cut, its only the governor that limits the fuel availably ( on early diesels ) modern electronics can be different, so with your older engine, increased air flow will not be a problem, more to the point less restrictive are flow will not be a problem and cooler ducted air even better.

Mark (mhpautos)

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Sunday, April 26th, 2009 AT 6:06 AM

SCOTSHIGHLANDER

MEMBER
3 POSTS

Thanks again for your words of wisdom!
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And, yes, I am aware of the operational characteristics of diesel engines regarding the governor.
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As I considered matters further, it occurs to me that Nissan also makes (or at one time made) a 2.2 Liter (4 cyl) diesel which is the direct forerunner of the 3.3 L. Engine.
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Essentially the 3.3 is the exact same design but with 2 more cylinders.
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Now, I going to think that the engineers got lazy and decided to use all the same parts as were used on the the 2.2 Liter design in order to save money, which I suppose is just fine to a point.
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Knowing engineers as I do, one or more of them likely thought: "Well, it's only two more cylinders, why worry?
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But I see your point regarding the 'swept volume.'
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And once again, thank you for your time!
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Sunday, April 26th, 2009 AT 2:15 PM